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IC 9124 



Bureau of Mines Information Circular/1987 



Mined Land Subsidence Impacts on Farmland 
With Potential Application to Illinois: 
A Literature Review 



By David L. Veith 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9124 






Mined Land Subsidence Impacts on Farmland 
With Potential Application to Illinois: 
A Literature Review 



By David L. Veith 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




jut- q ^ 



Library of Congress Cataloging in Publication Data: 



Veith, David L. 

Mined land subsidence impacts on farmland with potential applica- 
tion to Illinois. 

(Information circular ; 9124) 

Bibliography: p. 15—16. 

Supt. of Docs, no.: I 28.27: 9124. 

1. Subsidences (Earth movements)— Illinois. 2. Coal mines and mining— Illinois. 
3. Land use— Illinois. 4, Farms— Illinois. I. Title. II. Series: Information circular (United 
States. Bureau of Mines) ; 9124. 



TN295:U43 



[QE600.3.U6] 



622 s [333.76'16] 



86-600227 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Geological and land considerations 3 

Geology of the Illinois Basin 3 

Drainage classes of soils 3 

Land use qualifications 4 

Subsidence characteristics 6 

High-extraction mining 6 

Illinois mining 7 

Ground settlements of high-extraction mining 8 

Subsidence effects 9 

Western Pennsylvania ground water 9 

Illinois ground water 10 

Farmlands 10 

Crops 11 

Economic considerations 12 

Bureau of Mines and State of Illinois subsidence research program 13 

Summary and conclusions 14 

References 15 

ILLUSTRATIONS 

1 . Drainage classes of soils 4 

2. Optimized high-extraction underground coal mining plan 7 

3. Typical subsidence from single-panel high-extraction mining in western 

Pennsylvania 10 

TABLES 

1. Crop yields on three Virgin Island soil types with slope and erosion* with 

average to high levels of management 5 

2. Development of underground coal mining techniques in Illinois 7 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS 


REPORT 


ft 


foot pet 


percent 


in 


inch st 


short ton 


m 2 


square meter vol pet 


volume percent 


mi^ 


square mile yr 


year 


mm 


millimeter 


degree 



MINED LAND SUBSIDENCE IMPACTS ON FARMLAND 

WITH POTENTIAL APPLICATION TO ILLINOIS: 

A LITERATURE REVIEW 

By David L. Veith 1 



ABSTRACT 

This report summarizes a Bureau of Mines review of selected literature 
■on the effects of subsidence due to high-extraction underground coal 
mining on farmland areas. The data are presented for consideration in 
evaluating the subsidence effects due to similar mining techniques on 
the prime farmland areas of Illinois. 

In Illinois the ground water level in the glacial drift is only 2 to 
15 ft below the nearly flat to gently rolling ground surface. The main 
subsidence concern is flooding, as caused by the lowering of the ground 
surface, and the subsequent effects on crop production. Positive drain- 
age must be maintained in the farmlands, and the need exists for exten- 
sive regional planning by the mining and agricultural industries to 
maximize short- and long-term production from both efforts. The Bureau 
is cooperating with the State of Illinois to develop basic information 
necessary for such planning. 



1 Mining engineer, Twin Cities Research Center, Bureau of Mines, Minneapolis, MN, 



INTRODUCTION 



This report summarizes selected litera- 
ture dealing with underground mine- 
related subsidence effects on coal mining 
lands throughout the United States. Some 
foreign data are also summarized. The 
information is presented for consider- 
ation in the evaluation of subsidence ef- 
fects on Illinois prime farmland crop 
production, but in no way is it implied 
that the information pertains directly to 
Illinois unless the reference is clearly 
identified as such. 

As will be illustrated, the effects of 
underground coal mining subsidence in Il- 
linois have been investigated for some 
time, but no formal study of the effects 
on prime farmland crop production have 
been conducted. Efforts underway at this 
time will document such impacts and lead 
to mitigative measures that should opti- 
mize production from both the underground 
coal mining industry and the agricultural 
industry. 

Much of central and southern Illinois 
prime farmland overlies high-quality bi- 
tuminous coal seams. Maximization of 
underground coal production results in 
ground movements and subsequent distur- 
bance of the surface. Illinois ground 
water lies 2 to 15 ft below the ground 
surface, and subsidence-induced ground 
movements can also affect the movement 
of surface water. The ground water is 
generally dependent on precipitation 
to maintain its level and, therefore, 
its availability for crop production. 
Any modification of surface water hy- 
drology could affect the resultant crop 
production. 

The challenge is to maximize under- 
ground coal recovery in these Illinois 
prime farmland areas, and at the same 
time ensure the productivity levels of 
crops grown on the overlying soils. The 
solution lies in the cooperation of these 
two valuable industries to develop 
planned mining and agriculture programs 
to maximize recovery from both natural 
resources. To acquire the in-depth un- 
derstanding necessary to achieve this 
goal, the Bureau of Mines and the 
State of Illinois are cooperating on a 



coordinated mine subsidence research pro- 
gram to ultimately develop guidelines for 
more effective underground mining techni- 
ques in order to maximize coal extraction 
while preserving the productivity of 
prime farmlands. 

Illinois coal mining companies have 
basically three options for underground 
operations (1_):2 

1. Mine a low percentage of the coal 
and leave substantial pillars for total 
support of the overlying strata (low 
extraction). 

2. Mine a high percentage of the coal, 
plan on subsidence, and take the respon- 
sibility for reclamation of the affected 
surface (high extraction). 

3. Mine no coal at all (no 
extraction). 

Obviously there are certain circum- 
stances under which each of the three 
options would be the best choice. When 
considering maximum utilization of both 
the coal resource and the overlying prime 
farmland resource, option 2 is the best 
choice (1_)« High-extraction underground 
coal mining, planned in advance to mini- 
mize surface impacts on the overlying 
agriculture industry, and properly car- 
ried out, will result in the maximum 
utilization of both resources. 

In 1982, Peabody Coal Co. reported that 
193 subsidence events had occurred on its 
Illinois properties since 1971 (J L )« Less 
than 2 pet of its mined lands were af- 
fected. Subsidence regulation proposed 
at that time would have caused the 
company to lose about 70 million st 
of coal reserves (about 8 pet of their 
total reserves), and would have short- 
ened their mining life by more than 
42 yr, or more than 190-mine yr of 
production. Regulation that reduces 
mineable reserves because of potential 
subsidence impacts is, therefore, of 
great concern to mining companies such 
as Peabody. Research to improve the 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



cohabitation of the mining and agricul- 
tural industries in the prime farmland 
areas of Illinois is also of great 



interest to the involved mining compa- 
nies, the agricultural industry, and both 
State and Federal governmental agencies. 



GEOLOGICAL AND LAND CONSIDERATIONS 



GEOLOGY OF THE ILLINOIS BASIN (_2) 

Overall, the Illinois Basin is a broad, 
spoon-shaped trough of sediments con- 
taining several anticlinal belts. Gen- 
erally, the folds have gentle slopes with 
less than 200 ft of relief. Folding and 
faulting is most prominent in south- 
eastern Illinois. The topography ranges 
from flat-lying to rolling, with soils 
chiefly developed in glacial drift. Till 
deposits in the northern and central 
parts of the State reach 600 ft in depth, 
but the drift is generally less than 
50-ft deep in southern Illinois. 

Almost all the mapped coal reserves in 
Illinois are in the middle portion of the 
Pennsylvanian System, which underlies 
about 70 pet of the land of the State. 
Shales and underclays form 65 to 70 pet 
of the upper portions of the System, 
limestones form 5 to 10 pet, and coal 1 
to 2 pet. Shale beds exceed 100 ft in 
thickness but more commonly are 20-to-40 
ft thick, and the underclays are commonly 
2- to 5-ft thick. The Pennsylvanian 
sequence shows great lateral persist- 
ence, except for the sandstones, which 
frequently occur as channel deposits. 

DRAINAGE CLASSES OF SOILS (3) 

It is important in the study of the ef- 
fects of mine subsidence on prime farm- 
land to understand the drainage classes 
of soils and how these affect crop pro- 
duction. This understanding is par- 
ticularly important in Illinois where 
ground water is shallow and essentially 
contained in glacial overburden, sup- 
ported by shale overlying the coal seams. 

Figure 1 depicts the five U.S. Depart- 
ment of Agriculture (USDA) (h) drainage 
classes of soils and indicates approxi- 
mate root penetration with respect to the 
mottled zone in each class. A. mottled 
zone contains spots of different colors 
in a uniform soil matrix, indicating 



intermittent wetness with associated 
oxidized and reduced materials. 

In well-drained soils the moisture is 
rapidly removed by internal drainage. 
These soils often are dry because they 
are close to bedrock, are on very steep 
slopes, or have coarse textures that 
limit the moisture holding capacity. 

In moderately well drained soils the 
water is removed less rapidly than in 
well-drained soils, so that the profile 
is occasionally wet for short but signif- 
icant periods. These soils usually have 
low-permeability layers in the B or C 
horizons, and accumulations of seasonal 
seepage waters. 

Somewhat poorly drained soils are wet 
for significant periods but not continu- 
ously, as water is removed slowly. These 
soils have a low-permeability layer, a 
high water table, and accumulate water 
through seepage. 

Poorly drained soils remain wet much of 
the year because water is removed very 
slowly. The water table is near the sur- 
face much of the year; there is a low 
permeability layer, and seepage is a 
problem. 

Finally, very poorly drained soils are 
continuously wet, as the water table is 
near the surface most of the year. These 
soils are in depressions and ponding 
sites where water accumulates. 

According to the USDA, the rooting zone 
basically stops at the mottled zone, and 
the depth of this zone determines crop 
production. Changes in surface elevation 
with respect to the ground water eleva- 
tion may change the location of the mot- 
tled zone, and, therefore, the crop pro- 
duction capacity of the soil. 

If wet soils are permeable, they can be 
improved. When wet soils are only 
slightly permeable, drainage and amelio- 
ration are often difficult and productiv- 
ity improvement may only be slight. 

Olson (3) analyzed information from 
another USDA rating of soils for plant 



.Crop root 
development 



x" 

i- 

0_ 
LU 

Q 2 



3 L 




MWD 



SPD 



VPD 



Soil profile drainage classification 



Drainage class designation 
WD - Well drained 



Zone designation 
A - Rooting zone 



MWD - Moderately well drained B - Mottled zone 

SPD - Somewhat poorly drained C - Gray, brown, or stained zone 



PD - Poorly drained 
VPD - Very poorly drained 



V - Permanent or fluctuating water table 
(inferred from zone characteristics) 



FIGURE 1 .—Drainage classes of soils. After Olson (3). 



growth (5). Good plant growth requires 
well-drained to moderately well drained 
soil, with very friable to friable, moist 
consistency. Textures can be very fine 
sandy loam, fine sandy loam, loam, silt, 
or silt loam. The thickness of the soil 
above a hard layer, water table, or bed- 
rock must be more than 30 in. Finally, 
the soil slope must be less than 8 pet, 
and it must contain less than 8 vol pet 
coarse fragments. Fragments are consid- 
ered coarse depending on their shape and 
dimensions. Granular and crumb struc- 
tures are coarse if their particle diam- 
eter ranges from 5 to 10 mm, and angular 
to subangular blocky structures are 
coarse if their particle diameter ranges 
from 50 to 100 mm. 



LAND USE QUALIFICATIONS 

Olson (3) describes some physical char- 
acteristics of Tompkins County, NY, soils 
related to land use. Some of these char- 
acteristics can be identified with subsi- 
dence effects and, therefore, may have 
some application to Illinois. The county 
soil map shows that — 

1. More than 75 pet of active pasture 
and cropland have slopes of less than 
10 pet. 

2. About 80 pet of abandoned farmland 
has slopes of more than 10 pet. 

3. About 40 pet of forest land has 
slopes of 35 to 70 pet. 



4. About 75 pet of pasture land has 
soils with seasonal water tables more 
than 5 in below the surface. 

5. About 60 pet of the cropland has 
soils of moderate to rapid permeability. 
Thus, it appears that the most pro- 
ductive soils have slopes of less than 
10 pet, seasonal water tables (pasture 
land) more than 5 in below the surface, 
and moderate to rapid permeability 
drainage. 

Erosion of soils is probably the most 
destructive process that reduces the pro- 
ductivity of the land (3). Topsoils and 
the A horizon generally contain the most 
nutrients and the best structure for 
plant growth, and any material eroded 
from these parts of the soil profile will 
have a detrimental effect on crop yield 
and plant growth. 

A common agricultural tool, the Univer- 
sal Soil Loss Equation, permits planners 
and farmers to predict the average rate 
of soil erosion for each feasible alter- 
native combination of crop management 
related to a specific soil map unit, the 
rainfall pattern, and the topography. As 
described by Olson, (3), the Universal 
Soil Loss Equation is — 

A - RKLSCP 

where A = the computed soil loss per 
unit area, 

R = the rainfall and runoff 
factor, 

K = the soil erodability factor, 

L = the slope length factor, 

S = the slope steepness factor, 

C = the cover and management 
factor, 



and 



P = the support practice factor. 



Soil is protected from erosion by veg- 
etation, the design of which is deter- 
mined by the slope and contour of the 
land. Since erosion of soils is a nat- 
ural geomorphic process that can never be 
eliminated completely, tolerable soil 



losses can be established for each soil 
type to balance the soil building process 
and thus to maintain acceptable agricul- 
tural production over a long period of 
time. 

Olson (3) describes crop yields in the 
Virgin Islands for clay and clay-loam 
soil types of various slopes, with aver- 
age and high levels of management 
(table 1). Obviously, slopes greater 
than 5 pet and erosion are detrimental to 
crop production, resulting in crop losses 
ranging from 8 to 42 pet, when compared 
with production from relatively flat, un- 
eroded soils. Although probably not ap- 
plicable to the prime farmlands of Illi- 
nois, this comparison is included to 
illustrate the potential impact of slope 
and erosion on crop production. 

In Illinois prime farmlands, sub- 
sidence-created slopes are generally 
less than 1 pet but may reach 4 pet for 
short distances along their profile. 
Therefore, it appears that If subsidence 
from high-extraction underground coal 
mining creates surface slopes of less 
than 5 pet, the impact will not be great 
enough to significantly increase erosion 
of the soil. Erosion is not the concern 
in Illinois, but changes in drainage 
of the flat prime farmlands caused by 
subsidence are of concern. 

Sarkar (6_) identified 22 independent 
soil characteristics from which soil 
scientists attempt to correlate yields. 
These characteristics are representative 
of a broad range of soils; many are 
related to moisture holding capacity and 
thus are critical to plant growth and 
crop production. 

TABLE 1. - Crop yields on three Virgin 
Island soil types with slope and 
erosion, with average to high 
levels of management 



Soil type 
Clay , 

Clay-loam 1. . 
Clay-loam 2. . 
1 Eroded. 



Slope, pet 


Yield, pet 


0- 2 


100 


2- 5 


83- 90 


1 5-12 


58- 89 


2-15 


100 


1 5-12 


75- 92 


0- 3 


100 


5-12 


75- 92 



Some of Sarkar's characteristics, as 
described by Olson, which may have ap- 
plication in Illinois, follow: 

1. Yields on the same field in Mary- 
land clearly showed the effects of 
moisture and erosion. 

2. Iowa corn yields related directly 
to soil variables such as slope, bio- 
sequence, available water holding capac- 
ity, erosion, organic carbon, drainage 
class, clay content, bulk density, pH, 
available phosphorus, and available 
potassium. 

3. Soil factors most highly correlated 
with corn yields in North Carolina were 
moisture holding capacity, clay and sand 
combinations, extractable phosphorus, 
percent base saturation and related 
properties that control soil acidity, and 
the charge on the cation-exchange 
complex. 

Weather and agricultural management are 
major variables influencing the soils and 
the resultant yields. Since weather can- 
not be controlled, agriculture practices 
and mining activities affecting those 
practices become the controlling varia- 
bles. Soils determine the potential for 
moisture storage, which affects a soil's 
ability to withstand drought. Soil 
factors influencing runoff and erosion 
directly influence crop yields, and 



managing mining activities such that 
these soil factors are not adversely af- 
fected will ensure that crop yields can 
be maximized with proper aricultural 
practices. 

Olson (3) described a yield measuring 
procedure called sequential testing, 
which may be beneficial in measuring the 
effects of subsidence on crop production. 
Similar soils exhibit similar character- 
istics in the same geologic conditions. 
Sequential testing is the sampling of 
soils across a landscape in which the 
soils occupy "sequential" positions along 
the sampling transect. For example, 
soils in a humid region commonly have the 
same drainage sequence in similar geo- 
logic materials. By sampling the soils 
along a transect, soil drainage classes 
can be identified and the growth of 
moisture-sensitive vegetation can be mea- 
sured at each sample point. Thus, veg- 
etative growth can be related to the 
drainage class and to the soil depth to 
mottling and seasonal water tables. 

Similar sequences can occur for soil 
texture, slope, pH, fertility, land use, 
crop growth, crop yield, and other soil 
and land characteristics. If experiments 
are run on contrasting soils, one can 
evaluate the effects of soil differences 
on crop production. 



SUBSIDENCE CHARACTERISTICS 



HIGH-EXTRACTION MINING 

Peng (_7) summarized the mechanics of 
surface subsidence in the Appalachian 
Coal Basin due to underground coal mining 
in a 1980 publication. In very general 
terms, room-and-pillar mining with full 
pillar extraction can result in subsi- 
dence similar to that resulting from 
longwall mining. The author sized the 
maximum caving dome height at 30 to 50 
times the mining height with caving 
angles between 15° and 35° and an average 
angle of 25°. Ideally, strata overlying 
the coal seam above the caving zone will 
remain more or less intact and sag uni- 
formly, as would a continuous beam, re- 
sulting in a symmetrical surface sag 
about the centerline of the single panel. 
Actually, strata behavior is greatly 



dependent on the characteristics of its 
components, and the resulting sag may 
range from uniform to highly irregular. 
Multiple panels will result in overlap- 
ping subsidence profiles, skewed toward 
the previously mined panel. 

Controlling factors of surface subsi- 
dence discussed by Peng are well known 
and will not be repeated here. In the 
discussion on the prevention of subsi- 
dence impacts the point was made that ad- 
vance planning is the best method of 
eliminating or reducing surface ground 
damage. Regional planning of full- 
extraction operations, with complete re- 
gard for maintaining the relative posi- 
tion of surface contours, appears to be 
the best method of ensuring positive sur- 
face drainage. A hypothetical example of 
such a plan is shown in figure 2. 




Surface drainage 
system 

Surface contour 
elevation 

Direction of mining 
advance 



FIGURE 2.— Optimized high-extraction underground coal mining plan. 



ILLINOIS MINING 

Bauer (8_) presented a paper in 1981 
describing some of the characteristics of 
subsidence from various coal mining tech- 
niques in Illinois. Table 2 presents a 
modified list from Bauer of the progres- 
sion of mining methods and the related 
percentages of panel extraction commonly 
practiced in Illinois. High-extraction 
retreat and longwall are the methods gen- 
erally used in Illinois and are probably 
the methods of the future where subsi- 
dence is either desired or allowed. 
Where subsidence is not allowed, blind or 
checkerboard roora-and-pillar techniques 
are employed. 

Illinois subsidence takes two basic 
forms, pit subsidence and sag-trough sub- 
sidence. Pit subsidence is a sudden drop 
in the surface such that the hole has 



nearly vertical walls. It is caused by a 
sudden collapse of the bedrock over a 
mine void, usually where the mining was 
close to the surface and the overlaying 
strata were not competent. 

TABLE 2. - Progression of underground 
coal raining technique development 
in Illinois 

Panel extrac- 
Mining method tion, pet 

Roora-and-pillar: 

Early 1 < 80 

Modified < 80 

Blind 50- 65 

Checkerboard 40- 60 

Longf ace 100 

High-extraction retreat.... 70- 90 

Longwall 100 

1 If pillars pulled. 



Sag subsidence refers to a large, near- 
ly equidimensional subsidence depression 
over room-and-pillar mines. Trough sub- 
sidence is similar to sag subsidence, 
except that the depression is elongated 
and typically results from high- 
extraction room-and-pillar and longwall 
mining techniques. 

Data generated for Illinois by Bauer 
(jJ) indicate that pit subsidence develops 
only over shallow mines less than 165-ft 
deep. Sags or sags and troughs develop 
over both shallow and deep mines, and 
pits, sags, or troughs can develop over 
mines 75-to-165 ft deep. 

Of the possible factors affecting the 
subsidence profile characteristics with a 
given mine geometry, the regional geology 
seems to have the greatest influence (8). 
Room-and-pillar mining in central Il- 
linois is in the No. 5 coal seam, which 
is fairly thin and strong, resulting in 
strong pillars and, low height-to-width 
pillar ratios and, therefore, minimal 
subsidence due to pillar crushing or 
compression. 

Similar mining techniques are practiced 
in southwestern Illinois in the Herrin 
(No. 6) coal seam, which is thicker than 
the No. 5 seam and has some of the high- 
est quality mine roof in the State be- 
cause of well-developed, thick limestone 
beds overlying the coal (_8). Subsidence 
in this region is somewhat greater than 
that in central Illinois. 

Room-and-pillar mining in southern Il- 
linois, also in the Herrin (No. 6) coal 
seam, is in an area of major regional 
geological structures, with little or no 
limestone in the mine roof (8). Subsi- 
dence is greater in this region than in 
the two regions described above. 

In comparing subsidence in Illinois 
with that in Great Britain, Bauer (8) 
states that the angle of draw for Il- 
linois longwall mining is 12° to 25°, 
which is generally less than that for 
British operations. Probable causes of 
this difference are that (1) Illinois 
overburden rock is stiffer and more 
resistant to subsurface movements, 
(2) British rock, unlike the rock in Il- 
linois, has been subjected to tectonic 
faulting, and (3) multiple-seam mining 
is commonplace in Britian and not in 



Illinois. Multiple-seam operations tend 
to break up the overburden, making it a 
less stiff rock mass, and thus inducing 
more widespread subsidence. 

GROUND SETTLEMENTS OF HIGH- 
EXTRACTION MINING 

Generally, retreat room-and-pillar and 
longwall mining methods are similar 
enough to result in similar subsidence 
patterns where the geology is the same 
(2^). Longwall mining removes 100 pet of 
the coal within the panel; retreat room- 
and-pillar mining usually removes as much 
of the coal as local conditions allow, 
and when this removal is a high percent- 
age of the in-place coal, the resulting 
caving and ground movement is similar to 
that for longwall mining of nearly equal 
dimensions. However, care must be exer- 
cised in this analogy, as the rock 
loading sequence differs greatly between 
the two methods. 

At the Old Ben No. 24 Mine near Benton, 
IL, a longwall demonstration study was 
conducted under Bureau contract (2, 9)» 
Three longwall panels were driven adja- 
cent to a modified retreat room- 
and-pillar panel, and comparisons were 
made of their subsidence profiles. Long- 
wall panel 1 was first developed adjacent 
to the narrow room-and-pillar panel; 
then, in sequence, longwall panels 2 and 
3 were developed along panel 1. 

Retreat room-and-pillar mining (2_) at 
Old Ben generally recovered 80 to 90 pet 
of the coal; however, because of the 
proximity of the longwall panels, a some- 
what different retreat room-and-pillar 
mining method was required for the demon- 
stration section, which reduced recovery 
to less than 80 pet. Chain pillars about 
86 ft wide separated the previously mined 
retreat room-and-pillar panels from long- 
wall panel 1. The coal seam was 620-ft 
deep, and longwall panels 460-ft wide and 
7-ft thick were mined. Maximum settle- 
ment observed was 4.3 ft over longwall 
panel 1 and 2.9 ft over the retreat room- 
and-pillar panel during the measurement 
period (9). Retreat room-and-pillar and 
longwall raining methods are considered 
high-extraction techniques, but their 
recovery factors and subsidence profiles 



generally are not the same. In this case 
at Old Ben, with lower than normal re- 
treat room-and-pillar mining recovery, 
subsidence over that operation was ex- 
pected to be less than over the longwall 
panel. The stumps and fenders of coal 
left in place in the retreat room- 
and-pillar mining helped support the roof 



and, although they were readily crushed 
by the overburden weight, they helped 
prevent the occurrence of full closure. 
Additionally, the proximity of the re- 
treat room-and-pillar panel to the long- 
wall panel was expected to skew the sub- 
sidence profile across the longwall 
panel. Both expectations were realized. 



SUBSIDENCE EFFECTS 



WESTERN PENNSYLVANIA GROUND WATER 

SMC Martin Inc. ( 10 ) studied the ef- 
fects of coal mine subsidence on ground 
water aquifers in western Pennsylvania 
under Bureau contract. Although the 
structure and hydrological regime are 
quite different than in Illinois, this 
study is still of interest in describing 
the effects of undergound coal mine sub- 
sidence on the surface and on crop 
productivity. 

In western Pennsylvania, deep coal min- 
ing operations often affect regional 
ground water supplies through subsidence 
and the resulting fracturing of overlying 
strata. The existing ground water flow 
system is then disrupted as it preferen- 
tially flows toward the mine workings 
along the newly created or widened frac- 
ture system. Additionally, more pore 
space is created in the strata by frac- 
turing, so the aquifer can store more 
water. . The water table in the vicinity 
of the mine becomes depressed or elimi- 
nated, thus creating a new hydrologic en- 
vironment. As the rate of subsidence de- 
creases, the fracture system begins to 
close due to settlement of the strata. 
With less direct flow paths to the mine, 
recharge exceeds discharge and the head 
increases until a new equilibrium is 
reached, which may or may not be the same 
equilibrium point as before mining, 
depending on the elastic properties of 
the strata layers. 

According to SMC, longwall mining of 
coal in western Pennsylvania causes a 
zone of caving that typically forms a 
dome 30 to 50 times the height of mining, 
with the overlying strata remaining in- 
tact but subject to sagging (fig. 3). As 
the sag develops, a subsidence trough, or 
profile, is produced at the surface, 



enclosing an area larger than that of the 
mined panel. The horizontal limits of 
the trough depend on the angle of draw, 
which includes the total area displaying 
subsidence effects. In western Pennsyl- 
vania the practical limit of the angle of 
draw is generally taken as 25°. In Il- 
linois the subsidence pattern resulting 
from high-extraction mining tends to 
exhibit a profile with greater slopes 
around the inflection point than that 
shown in figure 3. 

Maximum subsidence over a single panel 
occurs at the centerline, and the magni- 
tude depends on the width of the panel, 
the mined seam thickness, and the verti- 
cal distance between the seam and the 
surface. Where the seam is flat with no 
adjacent panels being mined, the subsi- 
dence profile generally is symmetrical 
about the centerline. Multiple panels 
result in composite subsidence profiles 
as the effects overlap and are partly 
additive. 

The response of the overlying aquifer 
system to longwall mining depends on 
site-specific variables, including depth 
to coal, thickness of coal removed, min- 
ing method, rock mechanics, site stratig- 
raphy, and aquifer properties. These 
variables also influence the amount and 
degree of subsidence. Rock mechanical 
properties determine both the degree of 
fracturing in the rock units and the re- 
compression or healing of the strata that 
allows water level recovery. Site stra- 
tigraphy and aquifer properties control 
the influence of aquitardal layers and 
types of aquifer systems. 

To determine hydrological changes due 
to mining operations, it is necessary to 
analyze data from water level measure- 
ments, aquifer tests, and borehole 
geophysics, and to interpret these data 



10 



Maximum subsidence 




/ \_Subsidence profile 
(final surface profile) 



Angle of draw 



Seam 
thickness 



Panel mining width- 
(Extracted panel) 



FIGURE 3.— Typical subsidence from single-panel high-extraction mining in western Pennsylvania. 



by comparing premining and postmining 
conditions. The range of ground water 
fluctuations is a function of the ability 
of surface infiltration to reach the 
ground water system and of the degree of 
hydraulic connection between shallow 
aquifer zones receiving the surface in- 
filtration and the deep aquifer zones 
representing the final repository of 
ground waters. 

ILLINOIS GROUND WATER 

In some instances, Illinois ground 
water is mainly contained in glacial 
overburden supported by shale strata. 
The shale is highly plastic and, if it is 
located above the caving zone over a 
high-extraction mining system, the entire 
strata should sag and settle without 
major fracturing. Thus, few channels may 
open to drain ground water to the lower 
strata and to the mine, and, except for a 
temporary depression, the ground water 
level should return and remain intact. 

In the case of pit subsidence, general- 
ly associated with roora-and-pillar mining 



close to the surface, the pit walls sink 
along highly fractured vertical planes. 
Ground water thus flows more easily into 
the mine through channels that may take 
much longer to heal than those formed 
under sag subsidence conditions. 

Deeper mining and the resulting sag or 
trough subsidence may have the associated 
problem of a depressed water table ac- 
centuated by water draining into the mine 
and to the clay mine floor. The clay 
floor absorbs moisture, weakens, and is 
then heaved by punching pillars. The net 
results are convergence of the floor and 
roof, more subsidence, and a greater im- 
pact on surface topography and the water 
table. 

The reaction of ground water to subsi- 
dence in Illinois is being studied under 
Bureau-sponsored research, and the 
information will be available upon 
completion. 

FARMLANDS (1_1) 

The effects of mine subsidence on farm- 
lands in general depend on the type of 



11 



crops, soil character, hydrology, topog- 
raphy, and other environmental factors. 
To evaluate such effects requires an ex- 
tensive data base that does not now 
exist, although studies are underway. 
Some potential effects are — 

1. Surface fissures creating paths to 
drain water from the topsoil, thereby 
causing drier surface soil conditions and 
erosion, which widens the cracks. 

2. Changes in ground slope. Steepen- 
ing slope increases flow velocity, which 
enhances surface runoff and erodibility. 
Slope reduction could lead to water- 
logging and ponding of the soil and 
eventual crop reduction. 

3. Changes in ground elevation or 
slope that may disrupt surface drainage 
and the hydraulic regime, thereby causing 
flooding or ponding, especially if 
natural topographical barriers to flow 
are lowered or the water table is shallow 
and underlain by an impermeable layer. 

4. Decreases in soil fertility due to 
modification of the subsurface hydrology 
that results from downward migration of 
ground water through cracks. 

5. Changes in ground water quality due 
to contact with toxic materials. 

6. Occurrences of sinkholes, which may 
upset the normal drainage system through 
accumulation or loss of water. 

The USDA (12) related soil erosion po- 
tential to the Universal Soil Loss 
Equation. It further identifies slopes 
ranging from "gently sloping" to "moder- 
ately sloping" as those most susceptible 
to water and wind erosion. These slopes 
are in the range of 5 to 8 pet. Slopes 
of 6 to 8 pet displayed gradually in- 
creasing crop losses, but the loss in- 
creased much more rapidly with steeper 
slopes. 

Agricultural lands are classified by 
Guernsey (13) in relation to mine subsi- 
dence as prime and nonprime farmlands. 
Prime farmland has "the best combination 
of physical and chemical characteristics 
for producing food, feed, forage, fiber 
and oilseed crops. It is the land that 
gives the highest agricultural yield with 
minimum input when managed according to 
modern farming methods." Generally, 
prime farmland soils have slopes of less 
than 8 to 10 pet. 



Basically, subsidence damage to agri- 
cultural lands may result in the loss of 
use or reduced productivity, but the ex- 
tent of this effect is very site specific 
and, therefore, difficult to quantify in 
general terms. Ground movement changes 
the soil environment and the soil build- 
ing process, and unless the movement is 
quite severe, it may take many years to 
detect the changes resulting from subsi- 
dence. As long as positive drainage is 
maintained and surface slopes remain gen- 
tle enough to prevent erosion beyond what 
is acceptable, the impact of mined land 
subsidence on prime farmland should be 
minimal (13). 

CROPS 

In an unpublished preliminary report on 
the effects of high-extraction coal- 
mine-induced subsidence on crop produc- 
tion, Darmody (14) describes the test 
areas in Illinois selected to compare 
high-extraction retreat and longwall min- 
ing methods, and the procedures used to 
determine the differences in results. 
Preliminary results indicate that 
longwall-caused subsidence is much more 
obvious than that of high-extraction 
retreat mining; it is more clearly marked 
on the surface and especially on level 
divides or in bottoms. 

Although both types of mining make pre- 
viously wet soils wetter, high-extraction 
retreat mining wet areas are more random 
and less well defined than longwall wet 
areas. The orientation of mining panels 
tends to affect the severity of subsi- 
dence effects, in that mine panels run- 
ning with natural surface drainage seem 
to cause less impact than those running 
across the drainage. 

A subsided panel can change the local 
base of a stream running across it caus- 
ing ponding. This impact is greatest in 
areas of subtle topography, where subsi- 
dence of only a foot or two can change 
the drainage pattern of the entire 
watershed. 

Weather significantly affects subsi- 
dence impacts. In a particularly wet 
year the subsided area may never dry 
sufficiently to support crop growth; 
however, in a dry year the subsided 



12 



surface may be able to support vegetation 
that normally would not survive. 

A general conclusion concerning design 
criteria for longwall panels is that they 
should be oriented parallel to natural 
drainage patterns in areas of subtle to- 
pography to maintain positive drainage 
and potentially reduce surface crop dam- 
age. High-extraction retreat subsidence- 
induced effects, however, appear too 
random to generalize this type of conclu- 
sion, and more investigation is necessary 
to develop an innovative premine plan- 
ning system to ensure acceptable drain- 
age, as was generalized with long- 
wall orientation. 

ECONOMIC CONSIDERATIONS 

Surveys indicate that the most serious 
problems resulting from subsidence are 
depressions or potholes that cause water 
to stand, and the disruption of drainage 
patterns to either underground tile or 
surface drainageways (15). As a result, 
reduced crop yields and, in some cases, 
cases, complete crop loss have been 
reported. 

Restoration of land productivity af- 
fected by subsidence is accomplished by 
four main methods: 

1. Restoring surface drainage is the 
most frequent method; it is most commonly 
accomplished by digging surface drainage 
ditches. 

2. Replacing broken underground tile. 

3. Use of mechanical equipment to fill 
depressions with soil from the surround- 
ing area. 

4. Hauling in dirt from outside the 
area to fill depressions. 

The soils in Illinois were formed 
under varying conditions and, therefore, 
have differences in structure, texture, 
and potential agricultural yields. Of 
those areas where coal mining is 
practiced, north central, east central, 
and central Illinois have the high- 
est potential corn yields under a 
high level of management, which most 
successful farm operators now use. 
West central Illinois has moderately 
favorable productivity indexes and 



potential corn yields under high level 
management. Southwestern and southern 
Illinois soils are less productive, and 
potential corn yields under a high level 
of management are, therefore, lower than 
in the other areas (15). 

Poor drainage, whether natural or sub- 
sidence induced, will affect the timing 
and opportunity to perform field tillage 
and planting operations. Delayed spring 
planting caused by standing water and wet 
conditions affects plant growth and crop 
yield; the extent of such impacts has not 
yet been determined and will require 
carefully planned observation and analy- 
ses. The final evaluation must include 
both analysis of the effect on annual 
yields of major crops and the long-term 
effect on productivity, and the conser- 
vation of soil for use by future 
generations. 

Case examples from Illinois described 
by Guither (15) show crop yields affected 
by subsidence ranging from 30- to 100-pct 
reduction in productivity, depending on 
the location, topography, and rainfall. 
An interesting note is that in dry years 
subsided areas may show no impact as lit- 
tle water collects in the depressions. 
In fact, in dry areas subsidence-induced 
depressions may produce more than the 
surrounding area as they do collect mois- 
ture. However, in wet years the depres- 
sion may remain saturated for the entire 
growing season and totally eliminate crop 
production. 

The costs of reduced land productivity 
due to mined land subsidence fall basi- 
cally into two catagories: (1) loss of 
crop productivity and (2) restoration of 
damaged land to full productivity. There 
are additional costs to the community 
such as damage to public facilities, loss 
in land values, and loss in taxes. 

It seems highly probable that, with the 
advance of high-extraction coal raining in 
Illinois, the total amount of surface 
land affected by subsidence will become 
significant. Detailed analysis now in 
progress by the State should establish 
that amount and evaluate crop production 
loss due to subsidence associated with 
underground coal raining techniques. 



13 



BUREAU OF MINES AND STATE OF ILLINOIS SUBSIDENCE RESEARCH PROGRAM 



Cooperative research between the Bureau 
and the State of Illinois <16) has been 
initiated to develop techniques of reli- 
able subsidence prediction for high- 
extraction retreat and longwall mining 
methods. If successful, such prediction 
techniques will allow the use of high- 
extraction mining, which will cause 
planned, immediate subsidence with mini- 
mal impact on surface crop productivity. 
The Bureau is currently involved in four 
areas: (1) analysis of geomechanical and 
subsidence profile data, (2) prediction 
and modeling of subsidence, (3) assess- 
ment of stability problems in mines, and 
(4) characterization of structural foun- 
dation response to subsidence. 

The Bureau is cooperating with the 
State of Illinois in developing data 
bases for geomechanical properties of the 
overburden and subsidence profile char- 
acteristics. The Bureau-sponsored re- 
search program will increase the data 
base available in the State for analyzing 
overburden response to high-extraction 
mining methods by monitoring the overbur- 
den and ground surface over an existing 
high-extraction mine. 

The development of prediction and mod- 
eling techniques will visually demon- 
strate what may happen under various min- 
ing and geological conditions in Illinois 
coal mines. Models can be excellent 
guides, and the ability to predict hydro- 
logical changes caused by surface eleva- 
tion changes is necessary for the plan- 
ning of mining operations to minimize 
impacts on surface drainage patterns. 

A computer program is being developed 
to model subsidence-related changes in 
slopes and elevations. Initially sched- 
uled to be based on previously monitored 
longwall panels from southern Illinois, 
the final model will incorporate data 
from a 10-mi 2 area to ensure that all 
programs in the model are functional. 
Ideally, the final model will select a 
compromise between (1) no raining with no 
subsidence and (2) full-extraction min- 
ing with maximum subsidence, such that 
positive drainage is maintained or 
reestablished after the surface has been 



stabilized and maximum crop production is 
assured. 

A research program is underway to mon- 
itor seven subsidence sags that have de- 
veloped during the past 20 yr over a 
room-and-pillar mine. These sags have 
affected surface drainage. The sags 
probably resulted from subsidence due to 
pillar punching and time-related roof and 
floor squeeze, and depressions gradually 
developed on the surface. The research 
is designed to determine why abandoned 
room-and-pillar mines in Illinois become 
unstable, collapse, and create subsi- 
dence-related problems such as impacted 
surface drainage. 

Another Bureau-sponsored research proj- 
ect, at Southern Illinois University at 
Carbondale, is designed to assist in the 
characterization of pillar strength and 
stability problems for improvements in 
the design of planned and unplanned sub- 
sidence. Combined with State-sponsored 
work of the physical and mechanical prop- 
erties of floor materials, this research 
will aid in pillar stability analysis for 
mine design. 

Mine planners using high-extraction 
methods need to know how long subsidence- 
related movement will continue so that 
repairs to structures and farmland will 
not be made until the area is stable. 
The Bureau is investigating the duration 
of ground movement from high-extraction 
mining in Illinois. Generally, when coal 
is extracted from an area in Illinois, 
caving and surface movement happen rap- 
idly. Most existing data indicate that 
95 pet of movement occurs within 90 days 
of mining, but because the caved rock 
continues to settle and compact under its 
own weight, small movements may occur for 
years after the raining front has passed. 
Acceptable limits of residual movement 
need to be established such that needed 
restoration can take place as quickly as 
possible. 

The University of Illinois at Urbana- 
Champaign ( 17 ) is researching the effects 
of planned subsidence resulting from 
high-extraction underground coal raining 
on subsequent agricultural suitability 



14 



and productivity. Mined and unmined 
lands are being surveyed, and the effects 
of subsidence on crop growth will be 
determined. 

Future Bureau and Bureau-sponsored re- 
search will include developing subsidence 
prediction methods for the mining and 
geologic conditions in Illinois. More 
accurate analytical methods need to be 
developed, and these must be based on an 



understanding of how geological factors 
affect the migration of strata movements 
from the mine itself to the resulting 
surface profile. Subsidence prediction 
along with more complete geomechanical 
data must ultimately form the basis for 
mine and mining plan guidelines that will 
minimize the impacts of subsidence on the 
ability of overlying prime farmlands to 
sustain maximum crop production. 



SUMMARY AND CONCLUSIONS 



Generally, the Illinois Coal Basin is a 
broad, spoon-shaped trough of sediments 
containing anticlinal folds of gentle 
slope with less than 200 ft of relief. 
The topography ranges from flat to roll- 
ing with glacial till soils ranging from 
600-ft deep in the northern and central 
parts to less than 50-ft deep in southern 
Illinois. 

Most of the mapped coal reserves are in 
the middle portion of the Pennsylvanian 
System, underlying the southern two- 
thirds of the State. Overlying shales 
are generally 20- to 40- ft thick, and 
underlying clays are only 2- to 5- ft 
thick. Ground water is contained in the 
glacial overburden to a level of 2 to 15 
ft below the surface and is highly 
dependent on surface recharge. 

Retreat room-and-pillar and longwall 
are the two high-extraction underground 
coal mining methods used in Illinois. 
Both methods are high recovery systems, 
and their resulting subsidence and sur- 
face profiles can be similar. 

Subsidence from underground mining in 
western Pennsylvania disturbs the over- 
lying ground water by opening fractures 
in the strata and allowing the water to 
drain into lower strata and ultimately 
into the mine. After mining, the over- 
lying structure generally heals with time 
due to settling and compression, and the 
ground water level is restored by re- 
charge. The water may or may not reach 
its former level, depending on the new 
hydrologic balance. 

The potential effects of high-extrac- 
tion mining subsidence on the ground 
water in Illinois may be somewhat dif- 
ferent. The shale layers above the cav- 
ing zone and the overlying strata are 



flexible and tend to sag over the mined- 
out area, with only minor fracturing. If 
the subsided area quickly heals itself, 
the ground water in the glacial till may 
be restored to its former level; and, 
since the normal level is close to the 
surface, the new water level may exceed 
the subsided ground level. The results 
could then be highly saturated soils and 
ponding conditions that would severely 
reduce crop production. 

Good plant growth requires well-drained 
to moderately well-drained soil with good 
soil moisture consistency and texture. 
There should be at least 30 in of soil 
above a hard layer, and the surface slope 
should be less than 10 pet. Most im- 
portantly in Illinois, seasonal water 
tables need to be far enough below the 
ground surface to allow for good root 
development. 

Erosion is the single process that is 
most destructive to a soil, as it reduces 
the soil's productivity by removing nu- 
trients and structure that are contained 
in the upper soil layers. Erosion is 
mainly dependent on the soil unit, rain- 
fall, and topography. Of those three 
factors, only topography (i.e., slope) is 
controllable and directly affectd by mine 
subsidence. High-extraction underground 
coal mining subsidence in Illinois gener- 
ally results in minor slope changes and, 
therefore, negligible changes in the 
erosion potential. The major impact is 
on drainage, and where the topography is 
extremely flat, a small change in slope 
may result in a change from positive to 
negative drainage. As long as positive 
drainage is maintained, however, a small 
change in slope should not severely 
affect crop production. 




The Bureau and the State of Illinois 
are cooperating on research to develop 
techniques for reliable subsidence pre- 
diction for high-extraction retreat room- 
and-pillar and longwall mining meth- 
ods in Illinois. Successful prediction 
techniques will allow the use of 



15 



high-extraction mining methods, resulting 
in planned, immediate subsidence with 
minimal impact on surface crop produc- 
tion. The cooperative program will 
result in mining plans that maximize both 
coal production and subsequent crop pro- 
duction in the prime farmland areas. 



REFERENCES 



1. Guither, H. D. , J. Hines, and 
R. Bauer. The Economic Effects of Under- 
ground Mining Upon Land Used for Illi- 
nois Agriculture (Univ. IL at Urbana- 
Champaign, Dep. Agric. Econ. , and the IL 
State Geol. Surv. ) IL Dep. Ener. and Nat. 
Resour. , Doc. 85/01, Feb. 1985, 185 pp. 

2. O'Rourke, T. D. , and S. M. Turner. 
Longwall Subsidence Patterns: A Review 
of Observed Movements, Controlling Param- 
eters, and Empirical Relationships. Sch. 
Civil and Environ. Eng. , Cornell Univ. , 
Ithaca, NY, Geotech. Eng. Rep. 79-6, Nov. 
1979, 82 pp. 

3. Olson, G. W. Soils and the Envi- 
ronment, A Guide to Soil Surveys and 
Their Applications. Dowden & Culver, 
Inc. , 1981, 178 pp. 

4. U.S. Department of Agriculture. 
Soil Survey Manual. Agric. Handbook. 18, 
1962 (reissued), 503 pp. 

5. . Guide for Interpreting En- 
gineering Uses of Soils. 1971, 87 pp. 

6. Sarkar, P. K. , 0. W. Bidwell, and 
L. F. Marcus. Selection of Character- 
istics for Numerical Classification of 
Soils. Proc. Soil Sci. Soc. America, 
v. 30, 1966, pp. 269-272. 

7. Peng, S. S., and S. L. Cheng. 
Prediction of Surface Subsidence Profile 
Due to Underground Coal Mining. Dep. 
Min. Eng., Coll. Min. and Ener. Res., WV 
Univ. , Tech. Rep. TR 80-5, Dec. 1980, 
46 pp. 

8. Bauer, R. A., and S. R. Hunt. Pro- 
file, Strain, and Time Characteristics of 
Subsidence From Coal Mining in Illinois. 
Paper in Proceedings of Workshop on Sur- 
face Subsidence Due to Underground Min- 
ing, ed. by S. S. Peng and M. Harthill 



(Morgantown, WV, 
Dep. Miner. Eng. 
Resour. , WV Univ. , Mar. 
218. 

9. Janes, J. 
Longwall Mining 
Ben Coal Co. ). 
v. 1, 105 pp. ; v 



Nov. 30-Dec. 2, 1981). 

, Coll. Miner, and Ener. 

1982, pp. 207- 



R. A Demonstration of 
(contract J0333949, Old 
BuMines OFR 86-85, 1983, 
2 (appendix), 372 pp. 

10. Pennington, D. , J. G. Hill, G. J. 
Burgdorf, and D. R. Price. Effects of 
Longwall Mine Subsidence on Overlying 
Aquifers in Western Pennsylvania (con- 
tract J0199063, SMC Martin Inc.). Bu- 
Mines OFR 142-84, 1984, 149 pp.; NTIS 
PB 84-236710. 

11. Singh, M. M. , and S. Bhattacharya. 
Proposed Criteria for Assessing Subsi- 
dence Damage to Renewable Resource Lands. 
Soc. Min. Eng. AIME preprint 84-391, 
1984, 7 pp. 

12. U.S. Department of Agriculture. 
Land-Capability Classification. Agric. 
Handbook 210, 1973, 21 pp. 

13. Guernsey, L. , P. Mausel, and 
J. Oliver. An Overview of the Factors 
Involved in the Restoration of Mined 
Prime Farmland. Committee on Interior 
and Insular Affairs, Subcommittee on 
Energy and the Environment. 96th Congr. , 
1st sess., serial 96-4, Mar. 1979, 
pp. 577-580. 

14. Darmody, R. G. , I. J. Jansen, and 
N. T. Patterson. Effects on High Extrac- 
tion Coal Mine-Induced Subsidence on 
Crop Production. Prelim. Rep. to the 
Illinois Mine Subsidence Research Program 
Advisory Committee, Aug. 29, 1985, 3 pp.; 
available from R. G. Darmody, Dep. Agron- 
omy, Univ. IL at Urbana-Champaign. 



15087 21' 



16 



15. Guither, H. D. The Economic Ef- 
fects of Subsidence From Underground 
Coal Mines on Agricultural Land in Il- 
linois (contract H0222010, Univ. IL at 
Urbana-Champaign). BuMines OFR 186-84, 
1984, 60 pp.; NTIS PB 85-109296. 

16. Wade, L. V., and J. J. Olson. 
Productivity, Safety, and Environment — A 
Coordinated Approach for Improving Coal 
Mining Technology. Paper in Proceedings 



of Ninety-Third Annual Meeting of the 
Illinois Mining Institute (Springfield, 
IL, Oct. 31-Nov. 1, 1985). IL Min. 
Inst., 1986, pp. 21-44. 

17. Bauer, R. A. (Illinois State Geo- 
logical Survey Division). Private com- 
munication, Nov. 4, 1985; available upon 
request from David L. Veith, BuMines, 
Minneapolis, MN. 



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